A process for treating an organic wastewater as described above is generally an activated sludge process, wherein organic substances are subjected to biodegradation with microorganisms in a bioreactor. Water in the tank of the bioreactor is sent to a final settling-tank, and then subjected to solid-liquid separation by gravitational sedimentation so that the resultant supernatant water is taken out as treated water. However, this process requires a long period of time for the gravitational sedimentation; it is therefore final settling-tank needs large space area. Thus, mainly in sewage treatment plants, in particular, wherein a large space area is not easily secured, a membrane-separation activated sludge process (membrane bioreactor process), wherein the final sedimentation tank is substituted with a separation membrane, is often employed.
This membrane bioreactor process is roughly classified into a submersed membrane bioreactor process, wherein a separation membrane is submersed inside a bioreactor and water in the bioreactor is directly filtrated, and a side stream type membrane bioreactor process, wherein a separation membrane is placed outside a bioreactor, and water in the bioreactor is circulated to the separation membrane while the water is filtrated therewith. In the submersed membrane bioreactor process, a separation membrane is submersed in a bioreactor; therefore, it is impossible to remove particles (foulant) captured on the membrane and having the size close to a membrane pore size from the bioreactor to the outside. For this reason, the process has a problem that it is difficult to keep the stability of the filtration performance of the membrane certainly.
On the other hand, in the side stream type membrane bioreactor process, at the time when the foulants accumulate on the separation membrane, these foulants can be removed from the membrane by backwashing. The resultant backwash effluent is usually returned to the bioreactor through a route for the circulation of water in the bioreactor, and then reused. In some cases, the foulants in the backwash effluent are in the form of pellets. However, after the pellets are returned to the bioreactor, the pellets are aerated and broken into original particles.
The foulants returned to the bioreactor are partially taken into flocs of the activated sludge therein. However, the amount of the foulants taken in the activated sludge flocs decreases in wintertime when the activity of the sludge is lowered. Thus, the foulants increase in the bioreactor. Therefore, when a backwash effluent is returned to the bioreactor, the stability of the membrane filtration performance becomes difficult to ensure in the same reason as in the submersed membrane bioreactor process.
Thus, as disclosed in JP-A-2004-249235, which is Patent Document 1, it is suggested to discharge a backwash effluent from a separation membrane placed outside a bioreactor to the outside. However, according to this method, the MLSS concentration in the bioreactor falls excessively, and the biodegradation ability of organisms in the bioreactor may be reduced. Additionally, in order to treat the backwash effluent discharged to the outside, it becomes necessary to locate another effluent treatment facility. Thus, the process has a problem of lack of practicability.
Patent Document 1: JP-A-2004-249235